OBJECTIVES: We assessed whether state Medicaid preferred drug lists are concordant with the World Health Organization's 2009 16th Essential Medicines List and with each other. We also characterized listed medicines by generic availability and appearance on treatment guidelines.METHODS: We derived generic availability and first-line treatment status from the US Food and Drug Administration's Orange Book and the 2004-2009 National Health Service National Institute for Clinical Excellence guidelines. We report characteristics of Essential Medicines List and preferred drug list (PDL)-only medicines and describe differences between medicines that are frequently and infrequently listed on PDLs.RESULTS: Only 6 of 120 Essential Medicines List medicines appeared on fewer than 50% of PDLs. PDL-only medicines (n = 249) were less likely than were Essential Medicines List medicines (n = 120) to have generic versions available (56% vs 76%) and to be first-line treatments (21% vs 41%). The content of PDLs was variable: 33% of medicines appeared on 80% to 100% of PDLs.CONCLUSIONS: Application of the essential medicines concept to Medicaid PDLs could reduce costs and provide more equitable and evidence-based health care to low-income patients in the United States.